Wound dressing materials

ABSTRACT

A wound dressing material comprising: a wound dressing carrier, N-acetyl cysteine or a salt or derivative thereof, and a stabilized ascorbate. Suitably, the stabilized ascorbate comprises an ascorbate-2-polyphosphate. Also provided are wound dressings comprising the materials, methods of treatment with the materials, and methods of making the materials.

The present invention relates to wound dressing materials comprising, incombination, N-acetyl cysteine or salts or derivatives thereof (NAC) anda salt or a derivative of ascorbic acid. The present invention alsorelates to the use of such materials for the treatment of wounds, inparticular chronic wounds.

N-acetyl cysteine (N-acetyl-3-mercaptoalanine,L-Alpha-acetamido-beta-mercaptopropionic Acid, N-acetyl-L-cysteine,hereinafter referred to as NAC) is a derivative of the naturallyoccurring amino acid N-cysteine. NAG is a sulfhydryl group donor and istherefore considered an antioxidant, and as such it is commonly used asa stabilizer compound on pharmaceutical preparations. NAC is alsoavailable in pharmaceutical compositions for the treatment ofparacetamol (acetaminophen) overdose, and as a mucolytic in thetreatment of dry eye syndromes, chronic bronchitis and cystic fibrosis.

WO95/00136 describes the use of NAC or pharmaceutically acceptable saltsor derivatives thereof for the topical or systemic treatment ofhyperkeratosis or disorders mediated by proteinases. The specificdiseases mediated by proteinases cited in this application are lichenplanus, bullous diseases and mouth ulcers. The compositions mayoptionally contain ascorbic acid as a preservative.

WO93/04669 describes compositions containing NAC and its derivatives forregulating skin wrinkles and/or skin atrophy. The compositions mayoptionally contain from 0.1% to 10% of ascorbic acid or salts thereof asan anti-oxidant/radical scavenger.

U.S. Pat. No. 4,708,965 describes the use of NAC and its derivatives forthe treatment of herpes eruptions and ulcers. This use is based on theability of NAC to interfere with leukotriene synthesis.

U.S. Pat. No. 4,724,239 describes treatment of chemical ulcers caused byleukotriene production, the treatment being effected with compositionscontaining NAC.

EP-A-0849951 describes the use of a N-acetyl cysteine or apharmaceutically acceptable salt or derivative thereof for the treatmentor prophylaxis of chronic wounds. However, there is no suggestion to usethe NAC in combination with ascorbic acid or a derivative thereof.

Ascorbic acid (Vitamin C) is a well known acidulent, vitamin, andantioxidant substance. The use of ascorbic acid in wound dressings hasbeen studied. See, for example, WO-A-0201954, U.S. Pat. No. 4,711,780 orEP-A-0172326.

SUMMARY OF THE INVENTION

The present inventors have found that NAC and ascorbic acid derivatives,in combination, exhibit synergistic properties that are likely to makethe combination especially suitable for use in wound dressings,especially for the treatment of chronic wounds.

Accordingly, in a first aspect the present invention provides a wounddressing material comprising; a wound dressing carrier, N-acetylcysteine or a salt or derivative thereof; and a stabilized ascorbate.

In a further aspect, the present invention provides a wound dressingcomprising a wound dressing material according to the invention.

In a further aspect, the present invention provides a method oftreatment of a chronic wound comprising applying thereto an effectiveamount of a dressing material according to the invention.

In a further aspect, the present invention provides a materialcomprising a wound dressing carrier, N-acetyl cysteine or a salt orderivative thereof; and ascorbic acid or a salt or derivative thereof,for use in the treatment of a chronic dermal wound.

In a further aspect, the present invention provides a method of making awound dressing material comprising the steps of: (a) dispersing one ormore medically acceptable polymeric materials, N-acetyl cysteine or asalt or derivative thereof, and ascorbic acid or a salt or derivativethereof in an aqueous solvent to form an aqueous dispersion; and (b)freeze-drying or solvent-drying the aqueous dispersion to produce thewound dressing material.

DETAILED DESCRIPTION OF THE INVENTION

The term “derivative” refers to a chemical derivative that exhibitssimilar activity to the parent compound, and/or to a pro drug that iscleaved in vivo to the parent, active compound.

Medically-acceptable salts of the active compounds herein include, butare not limited to, sodium, potassium, magnesium, calcium, lithium,rubidium strontium, aluminum, boron, silicon and zinc salts of theactive compounds, or salts formed with cations of nitrogenous bases suchas ammonia, D-glucosamine or ethylenediamine.

As used herein, “effective amount” means an amount of compound orcomposition sufficient to significantly induce a positive modificationin the condition to be treated, but low enough to avoid serious sideeffects (at a reasonable benefit/risk ratio), within the scope of soundmedical judgment. The safe and effective amount of the compound orcomposition will vary with the particular condition being treated, theage and physical condition of the patient being treated, the severity ofthe condition, the duration of the treatment, the nature of concurrenttherapy, the specific compound or composition employed, the particularmedically-acceptable carrier utilized, and like factors within theknowledge and expertise of the attending physician.

The first active component of the wound dressings according to thepresent invention is N-acetyl Cysteine (NAC), or any medicallyacceptable salt or derivative thereof, optionally including cysteineitself. However, suitably the NAC derivative is not cysteine. Mostsuitably this component of the composition consists essentially of NACitself. Suitable derivatives are described, for example, in WO95/00136and WO93/04669. For example, a medically acceptable derivatives ofWO93/04669 have Formula I:R¹—CO—NH—CH(COOR³)—CH₂—S—R²  (I)or a medically-acceptable salt thereof, wherein:

R¹ is selected from the group consisting of nil and a C₁-C₁₈ alkyl,preferably C₁-C₇, more suitably C₁-C₃, more suitably still C₁ alkyl.

R² is selected from the group consisting of nil, —H, C₁-C₁₈ alkyl andR⁴CO—; suitably H or C₁-C₁₈ alkyl; more suitably H. In one embodiment,R² is suitably a C₁-C₁₈ alkyl, more suitably C₁-C₇, more suitably C₁-C₃,more suitably still C₁.

R³ is selected from the group consisting of —H, and C₁-C₁₈ alkyl,suitably —H. In one embodiment, R³ is suitably a C₁-C₁₈ alkyl, moresuitably C₁-C₇, more suitably C₁-C₃, more suitably still C₁

R⁴ is a C₁-C₁₈ alkyl; suitably C₁-C₇; more suitably C₁-C₃; more suitablystill C₁.

In another embodiment, both R¹ and R² are nil and the carbonyl carbonand the sulfur adjacent R¹ and R², respectively, are covalently bondedto form a cyclic ring. Otherwise, both R¹ and R² are other than nil.

Suitably, the materials of the present invention comprise NAC itself, ora medically acceptable salt thereof.

The materials according to the present invention suitably comprise fromabout 0.1% to about 20% of the NAC or salts or derivatives thereof, moresuitably from about 1% to about 10%, for example from about 2% to about8%, typically about 6% by weight based on the total weight of thecomposition.

The second active component of the wound dressings according to thepresent invention is a stabilized ascorbate.

The formulation of ascorbic acid itself presents difficulties because itis easily oxidized. Upon storage or exposure to light, oxygen, moistureand/or high temperature, ascorbic acid undergoes rapid degradation. Itis unstable in aqueous solution, even under neutral pH and at roomtemperature. It also reacts with certain minerals, such as iron andcopper salts, which contribute to a more rapid degradation. Therefore,the materials of the present invention suitably are substantially freeof ascorbic acid itself, for example they suitably comprise less thanabout 1 wt. % of the free acid, and preferably also less than about 1wt. % of ascorbic acid salts.

In order to overcome the problem of the lack of stability of ascorbicacid and its soluble salts, the materials of the present inventionsuitably do not comprise such materials. Instead, the materials of thepresent invention comprise ascorbate derivatives that are more stable,referred to herein as “stabilized ascorbate”. Generally, the stabilizedascorbate exhibits lower antioxidant activity that free ascorbic acid,for example as determined by the rate at which is reduces the absorbanceof a 10⁻⁴M solution of diphenylpicrylhydrazyl (DPPH), or as measured byits ability to inhibit the oxidation of ABTS(2,2′-azino-di-[3-ethylbenzthiazoline sulphonate]) by a peroxidase.

According to one approach, stabilization of ascorbic acid and itsderivatives is achieved after entrapment into cyclodextrins, zeolites orliposomes.

Another approach consists of stabilizing ascorbic acid by derivatizationof its enediol function. Ascorbic acid in its lactone form has fourhydroxyl groups at carbons 2, 3, 5 and 6. These hydroxyl groups havedifferent chemical activities: the 2- and 3-hydroxyl groups, togetherwith the double bond connecting carbons 2 and 3, form an enediol systemthat is very sensitive to oxidation and is responsible for the oxidativedegradation of ascorbic acid, whereas the 5- and 6-hydroxyl groups forma rather stable-diol system. Common derivatization of ascorbic acidconverts the hydroxyl groups to alkyl-, acyl-, sulfo-, orphosphoryl-containing groups, which also affect the solubility ofascorbic acid in water or in oils. Known ascorbic acid derivatives fallinto two main groups, water-soluble and oil-soluble ascorbic acidderivatives. Stabilization of ascorbic acid by derivatization of itsenediol function at the 2-position has been attempted by preparation ofthe water-soluble derivatives L-ascorbic acid 2-phosphate or L-ascorbicacid 2-sulfate salts. For example, magnesium L-ascorbic acid-2-phosphate(MAP) is a water-soluble, stable, non-poisonous and non-irritatingderivative. Among the known oil-soluble derivatives the best known is6-ascorbyl palmitate. Suitably, the materials of the present inventioncomprise water-soluble ascorbic acid derivatives.

U.S. Pat. No. 4,647,672 and U.S. Pat. No. 5,149,829 describe stable,2-polyphosphorylated species of L-ascorbic acid and its stereoisomers.The 2-polyphosphate esters of L-ascorbate described in these patentshave proved to be an excellent source of vitamin C for nutrition,particularly in aquatic feeds, on account of their stability, lowsolubility and high bioavailability. The ascorbate 2-polyphosphateesters are commercially available from Roche under the Registered TradeMark STAY-C.

Suitably, the wound dressing material of the present invention comprisesan ascorbic acid derivative selected from ascorbate 2-phosphate orpolyphosphate compound, Trisodium-L-ascorbyl-2-monophosphate;2-Phospho-L-ascorbic acid trisodium salt, Magnesium Ascorbyl Phosphate(MAP), L-Ascorbic acid mono(dihydrogen phosphate) magnesium salt,Magnesium L-ascorbic acid-2-phosphate,trisodium-L-ascorbyl-2-polyphosphate, or a mixture thereof.

Ascorbate 2-polyphosphates (AZP) and derivatives thereof suitable foruse in the present invention have the Formula I:

where X and Y are different respectively taken from the group consistingof —H and —OH, and q normally ranges from 1 to 4; in addition A₁, A₂,A₃, A₄ and A₅ are respectively taken from the group consisting ofhydrogen and salt-forming cations. The materials of the presentinvention may comprise mixtures of more than one compound of Formula r.

The ascorbate 2-polyphosphates useful herein encompass not only the freeacid forms but also the salts thereof (e.g., alkali metal, alkali earth,ammonium or C1-C10 alkylammonium salts). The ascorbate may be anystereoisomer, or mixtures thereof. L-ascorbate 2-polyphosphates arepreferred. A suitable ascorbate 2-polyphosphate comprises thetriphosphate, suitably L-ascorbate 2-triphosphate. Suitably, at leastabout 50 wt. % of the ascorbate in the dressing is the triphosphate, andsuitably it consists essentially of the triphosphate. Suitable methodsof preparation and purification of the ascorbate 2-polyphosphates aredescribed in U.S. Pat. No. 4,647,672 and U.S. Pat. No. 5,149,829.

The materials according to the present invention suitably comprise fromabout 0.1% to about 20% of the stabilized ascorbate, more suitably fromabout 1% to about 10%, for example from about 2% to about 8%, typicallyabout 6% by weight based on the total weight of the composition.

Suitably, the weight ratio of the NAC or salts or derivatives thereof tothe stabilized ascorbate in the materials of the present invention is inthe range of from about 1:10 to about 10:1, more suitably from about 1:4to about 4:1, for example from about 1:2 to about 2:1. Suitably, themolar ratio of the NAC or salts or derivatives thereof to the stabilizedascorbate in the materials of the present invention is in the range offrom about 1:10 to about 10:1, more suitably from about 1:4 to about4:1, for example from about 1:2 to about 2:1.

Suitably, the materials according to the present invention providesustained release of the NAC and ascorbate into the wound in use.Suitably, the materials according to the present invention provide aconcentration of NAC (or derivatives or salts thereof) and ascorbate (orderivatives or salts thereof) in wound fluid of from about 0.1 mM toabout 10 mM, more suitably from about 0.5 mM to about 8 mM, for examplefrom about 1 mM to about 6 mM.

The term “wound dressing carrier”, as used herein, means one or morecompatible solid, semi-solid or liquid filler diluents ormicroencapsulating substances which are suitable for administration to ahuman or higher animal. Wound dressing carriers must be of sufficientlyhigh purity and sufficiently low toxicity to render them suitable foradministration to the human or animal being treated. A safe andeffective amount of carrier is suitably from about 50% to about 99% byweight, more suitably from about 80% to about 98%, for example fromabout 85% to about 95% by weight, of the material.

Suitably, the wound dressing carrier is bioabsorbable. The term“bioabsorbable” refers to a material that is fully degraded and absorbedin vivo in the mammalian body. Suitably, the materials comprise, orconsist essentially of, one or more biopolymers. That is to say,polymers of biopolymer origin, optionally chemically modified and/orcross-linked.

Suitably, the wound dressing carrier is a solid carrier. For example thesolid carrier may be a woven or nonwoven fabric, a foam, or afreeze-dried or solvent-dried sponge. In other embodiments, the solidcarrier may be a film of a medically acceptable polymer, such as ahydrogel polymer, optionally containing a plasticiser to improveflexibility. The active agents may be coated on, or dispersed in, thecarrier.

The carrier is usually not water soluble, but it may be water swellable.The polymers forming the carrier may be bioabsorbable ornon-bioabsorbable. Suitable non-bioabsorbable polymers includealginates. Suitable bioabsorbable polymers include those selected fromthe group consisting of collagens, bioabsorbable cellulose derivativessuch as oxidized celluloses, galactomannans such as guar or xanthan,glycosaminoglycans such as cross-linked hyaluronates, and mixturesthereof.

Suitably, the carrier in the wound dressing material of the inventioncomprises or consists essentially of a freeze-dried or solvent-driedpolymer sponge having the active agents dispersed therein.

In certain embodiments the polymeric sponge matrix comprises (and mayconsist essentially of) a solid bioabsorbable polymer selected from thegroup consisting of collagens, chitosans, oxidized celluloses, andmixtures thereof.

Oxidized cellulose is produced by the oxidation of cellulose, forexample with dinitrogen tetroxide as described in U.S. Pat. No.3,122,479. This process converts primary alcohol groups on thesaccharide residues to carboxylic acid group, forming uronic acidresidues within the cellulose chain. The oxidation does not proceed withcomplete selectivity, and as a result hydroxyl groups on carbons 2 and 3are occasionally converted to the keto form. These ketone unitsintroduce an alkali labile link, which at pH7 or higher initiates thedecomposition of the polymer via formation of a lactone and sugar ringcleavage. As a result, oxidized cellulose is biodegradable andbioabsorbable under physiological conditions.

A suitable oxidized cellulose for practical applications is oxidizedregenerated cellulose (ORC) prepared by oxidation of a regeneratedcellulose, such as rayon. It has been known for some time that ORC hashaemostatic properties, and that application of ORC fabric can be usedto reduce the extent of post-surgical adhesions in abdominal surgery.

Chitin is a natural biopolymer composed of N-acetyl-D-glucosamine units.Chitin may be extracted from the outer shell of shrimps and crabs inknown fashion. The chitin is then partially deacetylated, for example bytreatment with 5M-15M NaOH, to produce chitosan. Complete deacetylationof the chitin is not a practical possibility, but suitably the chitosanis at least 50% deacetylated, more suitably at least 75% deacetylated.Chitosan has been employed for wound treatment in various physicalforms, e.g. as a solution/gel; film/membrane; sponge; powder or fiber.Chitosan in the free base form is swellable but not substantiallysoluble in water at near-neutral pH, but soluble in acids due to thepresence of ammonium groups on the chitosan chain. The solubility of thechitosan may be reduced by cross-linking, for example withepichlorhydrin. Typically, the average molecular weight of the chitosanas determined by gel permeation chromatography is from about 105 toabout 106.

The collagen useful in the polymeric sponge materials according to thepresent invention may be any collagen, including Type I or Type II orType III collagen, natural fibrous collagen, atelocollagen, partiallyhydrolysed collagens such as gelatin, and combinations thereof. Naturalfibrous collagen, for example of bovine origin, is suitable. Forexample, the collagen prepared from bovine hide is a combination of TypeI collagen (85%) and Type III collagen (15%).

In certain embodiments of the present invention, the oxidized celluloseis complexed with collagen and/or chitosan to form sponges of the kinddescribed in WO98/00180, WO98/00446 or WO2004/026200. For example, theoxidized cellulose may be in the form of milled ORC fibres that aredispersed in a freeze-dried collagen or chitosan sponge. This providesfor certain therapeutic and synergistic effects arising from thecomplexation with collagen.

In particular embodiments, the polymeric sponge carrier comprises (andmay consist essentially of) a mixture of: (a) collagen and/or chitosan;and (b) oxidized regenerated cellulose, for example in a dry weightratio range of from about 90:10 to about 10:90 of collagen/chitosan:ORC,suitably from about 75:25 to about 25:75, and particularly from about60:40 to about 40:60.

The wound dressing material may also comprise up to 20% by weight,suitably less than 10% by weight of water. The material may also contain0-40% by weight, suitably 0-25% by weight of a plasticiser, suitably apolyhydric alcohol such as glycerol.

The material may also comprise 0-10% by weight, suitably 0-5% by weightof one or more therapeutic wound healing agents, such as non-steroidalanti-inflammatory drugs (e.g. acetaminophen), steroids, antibiotics(e.g. penicillins or streptomycins), antiseptics (e.g. silversulfadiazine or chlorhexidine), or growth factors (e.g. fibroblastgrowth factor or platelet derived growth factor). All of the abovepercentages are on a dry weight basis.

The preferred antimicrobial agent for inclusion in the wound dressingmaterials according to the present invention is silver (as silver ionsand metallic silver), suitably in an amount of from about 0.01 wt % toabout 5 wt. %, more suitably from about 0.05 wt % to about 1 wt. %, andmost suitably about 0.1 wt. % to about 0.3 wt. %. In preferredembodiments, the silver may be complexed to the polymeric carriermaterial. The term “complex” refers to an intimate mixture at themolecular scale, suitably with ionic or covalent bonding between thesilver and the polymer. The complex suitably comprises a salt formedbetween an anionic polymer or collagen and Ag⁺. Suitable wound dressingsponges comprising silver are described in more detail in WO2004/024197.

Suitably, the material according to the present invention will absorbwater or wound fluid and hence become wet, swell or become a gelatinousmass but will not spontaneously dissolve or disperse therein. That is tosay, it is hydrophilic but has a solubility of suitably less than about1 g/liter in water at 25° C. Low solubility renders such materialsespecially suitable for use as wound dressings to remove reactive oxygenspecies from the wound fluid.

The wound dressing material is typically in sheet form, for examplehaving an area of from about 1 cm² to about 400 cm², in particular fromabout 2 cm² to about 100 cm². The basis weight of the sheet is typicallyfrom about 100 g/m² to about 5000 g/m², for example from about 400 g/m²to about 2000 g/m².

The wound dressing material according to the present invention issuitably sterile and packaged in a microorganism-impermeable container.

In a further aspect, the present invention provides a wound dressingcomprising a wound dressing material according to the invention.

The wound dressing is suitably in sheet form and comprises an activelayer of the material according to the invention. The active layer wouldnormally be the wound contacting layer in use but in some embodiments itcould be separated from the wound by a liquid-permeable top sheet.Suitably, the area of the active layer is from about 1 cm² to about 400cm², more suitably from about 4 cm² to about 100 cm².

Suitably, the wound dressing further comprises a backing sheet extendingover the active layer opposite to the wound facing side of the activelayer. Suitably, the backing sheet is larger than the active layer suchthat a marginal region of width 1 mm to 50 mm, suitably 5 mm to 20 mmextends around the active layer to form a so-called island dressing. Insuch cases, the backing sheet is suitably coated with a pressuresensitive medical grade adhesive in at least its marginal region.

Suitably, the backing sheet is substantially liquid-impermeable. Thebacking sheet is suitably semipermeable. That is to say, the backingsheet is suitably permeable to water vapour, but not permeable to liquidwater or wound exudate. Suitably, the backing sheet is alsomicroorganism-impermeable. Suitable continuous conformable backingsheets will suitably have a moisture vapor transmission rate (MVTR) ofthe backing sheet alone of 300 to 5000 g/m²/24 hrs, suitably 500 to 2000g/m²/24 hrs at 37.5° C. at 100% to 10% relative humidity difference. Thebacking sheet thickness is suitably in the range of 10 to 1000micrometers, more suitably 100 to 500 micrometers. It has been foundthat such moisture vapor transmission rates allow the wound under thedressing to heal under moist conditions without causing the skinsurrounding the wound to macerate.

Suitable polymers for forming the backing sheet include polyurethanesand poly alkoxyalkyl acrylates and methacrylates such as those disclosedin GB-A-1280631. Suitably, the backing sheet comprises a continuouslayer of a high density blocked polyurethane foam that is predominantlyclosed-cell. A suitable backing sheet material is the polyurethane filmavailable under the Registered Trade Mark ESTANE 5714F.

The adhesive (where present) layer should be moisture vapor transmittingand/or patterned to allow passage of water vapor therethrough. Theadhesive layer is suitably a continuous moisture vapor transmitting,pressure-sensitive adhesive layer of the type conventionally used forisland-type wound dressings, for example, a pressure sensitive adhesivebased on acrylate ester copolymers, polyvinyl ethyl ether andpolyurethane as described for example in GB-A-1280631. The basis weightof the adhesive layer is suitably 20 to 250 g/m², and more suitably 50to 150 g/m². Polyurethane-based pressure sensitive adhesives arepreferred.

Further layers of a multilayer absorbent article may be built up betweenthe active layer and the protective sheet. For example, these layers maycomprise an absorbent layer between the active layer and the protectivesheet, especially if the dressing is for use on exuding wounds. Theoptional absorbent layer may be any of the layers conventionally usedfor absorbing wound fluids, serum or blood in the wound healing art,including gauzes, nonwoven fabrics, superabsorbents, hydrogels andmixtures thereof. Suitably, the absorbent layer comprises a layer ofabsorbent foam, such as an open celled hydrophilic polyurethane foamprepared in accordance with EP-A-0541391. In other embodiments, theabsorbent layer may be a nonwoven fibrous web, for example a carded webof viscose staple fibers. The basis weight of the absorbent layer may bein the range of 50-500 g/m², such as 100-400 g/m². The uncompressedthickness of the absorbent layer may be in the range of from 0.5 mm to10 mm, such as 1 mm to 4 mm. The free (uncompressed) liquid absorbencymeasured for physiological saline may be in the range of 5 to 30 g/g at25°. Suitably, the absorbent layer or layers are substantiallycoextensive with the active layer.

The wound facing surface of the dressing is suitably protected by aremovable cover sheet. The cover sheet is normally formed from flexiblethermoplastic material. Suitable materials include polyesters andpolyolefins. Suitably, the adhesive-facing surface of the cover sheet isa release surface. That is to say, a surface that is only weaklyadherent to the active layer and the adhesive on the backing sheet toassist peeling of the adhesive layer from the cover sheet. For example,the cover sheet may be formed from a non-adherent plastic such as afluoropolymer, or it may be provided with a release coating such as asilicone or fluoropolymer release coating.

Typically, the wound dressing according to the present invention issterile and packaged in a microorganism-impermeable container.

In a further aspect, the present invention provides method of treatmentof a wound comprising applying thereto an effective amount of a dressingmaterial according to the present invention. Suitably, the treatmentcomprises applying to said wound a dressing according to the presentinvention.

The material is suitably applied to the wound for a period of at leastabout 1 hour, more suitably from about 4 hours to about 4 weeks, forexample from about 1 day to about 14 days, optionally with dressingchanges during the course of the treatment.

Suitably, the wound is a chronic wound. More suitably, the chronic woundis selected from the group consisting of ulcers, in particular dermalulcers. For example, venous ulcers, decubitus ulcers, or diabeticulcers. The wound dressing materials according to the present inventionpromote collagen synthesis and collagen gel contraction in vitro (seebelow), which suggests that they may provide advantages in the healingof chronic wounds.

Accordingly, in a further aspect, the present invention provides amaterial comprising a wound dressing carrier, N-acetyl cysteine or asalt or derivative thereof; and ascorbic acid or a salt or derivativethereof, for use in the treatment of a chronic dermal wound.

Suitably, the carrier, the NAC and the ascorbate are as described abovein relation to the first aspect of the invention. Where present, theascorbic acid or salts thereof are used in similar amounts to theamounts of the stabilized ascorbate used in the embodiments according tothe first aspect.

In a further aspect, the present invention provides a method of making awound dressing material comprising the steps of.

-   -   (a) dispersing one or more medically acceptable polymeric        materials, N-acetyl cysteine or a salt or derivative thereof,        and ascorbic acid or a salt or derivative thereof in an aqueous        solvent to form an aqueous dispersion; and    -   (b) freeze-drying or solvent-drying the aqueous dispersion to        produce the wound dressing material.

Suitably, the method according to this aspect of the invention is amethod of making a wound dressing material according to the first orsecond aspects of the invention.

The polymeric materials are suitably as described above in relation tothe first aspect of the present invention. Suitably, the dispersion hasa solids concentration of from about 0.5% to about 3% by weight.

Suitably, the dispersion has a pH of from about 3 to about 4. Where aderivative such as ascorbate 2-polyphosphate that has low solubility areused, conventional acidifying agents such as acetic acid are used toachieve the desired pH.

Suitably, the dispersion comprises from about 0.0002% to about 1% byweight of the NAC, salt or derivative thereof, for example from about0.03% to about 0.4%, typically about 0.05% to about 0.3%, based on theweight of the dispersion. Suitably, the dispersion comprises from about0.0002% to about 1% by weight of the ascorbic acid, salt or derivativethereof, for example from about 0.03% to about 0.4%, typically about0.05% to about 0.3%, based on the weight of the dispersion.

The method according to this aspect of the invention further comprisesfreeze-drying or solvent-drying the dispersion. Freeze-drying comprisesthe steps of freezing the dispersion, followed by evaporating thesolvent from the frozen dispersion under reduced pressure. Suitably, themethod of freeze-drying is similar to that described for acollagen-based sponge in U.S. Pat. No. 2,157,224. Solvent dryingcomprises freezing the dispersion, followed by immersing the frozendispersion in a series of baths of a hygroscopic organic solvent such asanhydrous isopropanol to extract the water from the frozen dispersion,followed by removing the organic solvent by evaporation. Methods ofsolvent drying are described, for example, in U.S. Pat. No. 3,157,524.

In certain embodiments the process may further comprise treating thedispersion, or the dried material, with a cross-linking agent such asepichlorhydrin, carbodiimide, hexamethylene diisocyanate (HMDI) orglutaraldehyde. Alternatively, cross-linking may be carried outdehydrothermally. The method of cross-linking can markedly affect thefinal product. For example, HMDI cross-links the primary amino groups oncollagen, whereas carbodiimide cross-links carbohydrate on the ORC toprimary amino groups on the collagen.

Especially suitable methods of making freeze-dried and solvent-driedsponges are described in EP-A-1153622 and EP-A-0838491.

It will be appreciated that any feature or embodiment that is describedherein in relation to any one aspect of the invention may also beapplied to any other aspect of the invention.

Specific embodiments of the invention will now be described further, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a wound dressing according to the present inventionincorporating a sheet of the material according to the invention.

FIG. 2 shows the percentage stimulation of collagen synthesis in dermalfibroblast cells measured by Procedure 1 for the examples and referenceexamples.

FIG. 3 shows the observed radius of the collagen sheet in the gelcontraction measurement according to Procedure 2 for certain examplesand reference examples.

Referring to FIG. 1, a wound dressing 1 according to the presentinvention is an island-type, self-adhesive wound dressing comprising abacking layer 2 of microporous liquid-impermeable polyurethane foam. Thebacking layer 2 is permeable to water vapor, but impermeable to woundexudate and microorganisms.

The backing layer is coated with a substantially continuous layer ofpressure-sensitive polyurethane adhesive. A rectangular island 3 of awound dressing material according to the invention in sheet form, madein accordance with Example 1 below, is adhered to a central region ofthe adhesive-coated backing sheet 2 such that an adhesive-coated margin4 of the backing sheet extends around the island for attachment of thedressing to the skin around a wound.

The dressing further comprises protective, release-coated cover sheets5,6. These cover sheets are removed immediately before use of thedressing. The dressing is suitably sterile and packaged in amicroorganism-impermeable pouch (not shown) prior to use.

Example 1

A collagen/ORC sponge containing NAC and AZP was prepared by amodification of the method for the preparation of Collagen/ORC spongesdescribed in Example 1 of EP-A-1153622.

Briefly, the collagen component is prepared from bovine corium asfollows. Bovine corium is split from cow hide, scraped and soaked insodium hypochlorite solution (0.03% w/v) to inhibit microbial activitypending further processing. The corium is then washed with water andtreated with a solution containing sodium hydroxide (0.2% w/v) andhydrogen peroxide (0.02% w/v) to swell and sterilize the corium atambient temperature. The corium splits then undergo an alkali treatmentstep in a solution containing sodium hydroxide, calcium hydroxide andsodium bicarbonate (0.4% w/v, 0.6% w/v and 0.05% w/v, respectively) atpH greater than 12.2, ambient temperature, and for a time of 10-14 days,with tumbling, until an amide nitrogen level less than 0.24 mmol/g isreached. The corium splits then undergo an acid treatment step with 1%hydrochloric acid at ambient temperature and pH 0.8-1.2. The treatmentis continued with tumbling until the corium splits have absorbedsufficient acid to reach a pH less than 2.5. The splits are then washedwith water until the pH value of corium splits reaches 3.0-3.4. Thecorium splits are then comminuted with ice in a bowl chopper first witha coarse comminution and then with a fine comminution setting. Theresulting paste, which is made up in a ratio of 650 g of the coriumsplits to 100 g of water, as ice, is frozen and stored before use in thenext stage of the process. However, the collagen is not freeze-driedbefore admixture with the ORC in the next stage.

The ORC component of the freeze-dried pad is prepared as follows. ASURGICEL cloth (Johnson & Johnson Medical, Arlington) is milled using arotary knife cutter through a screen-plate, maintaining the temperaturebelow 60° C.

The milled ORC powder and the required weight (according to solidscontent) of frozen collagen paste are then added to a sufficient amountof water acidified with acetic acid to form an aqueous dispersion. NACand Ascorbate 2-triphosphate (STAY-C, Roche) are each dissolved into theaqueous acetic acid prior to addition of the ORC and collagen, to give afinal concentrations of NAC of 4.5 mM and a final concentration ofascorbate 2-triphosphate of 4.5 mM. The resulting aqueous dispersion haspH value of 3.0 and a total solids content of 2.0% (note: the method ofExample 1 of EP-A-1153622 uses a 1% solids slurry). The mixture ishomogenized through a Fryma MZ130D homogenizer, progressivelydiminishing the settings to form a homogeneous slurry. The pH of theslurry is maintained at 2.9-3.1. The slurry temperature is maintainedbelow 20° C., and the solids content is maintained at 2%±0.07.Surprisingly, it was found that the slurry having this higher solidscontent has a sufficiently low viscosity for handling in the subsequentstages of the process.

The resulting slurry is pumped to a degassing vessel. Vacuum isinitiated for a minimum of 30 minutes, with intermittent stirring, todegas the slurry. The slurry is then pumped into freeze-drier trays to adepth of 25 mm. The trays are placed onto freezer shelves where thetemperature has been preset to −40° C. The freeze-drier programme isthen initiated to dry and dehydrothermally cross-link the collagen andORC to form thick sponge pads. On completion of the cycle, the vacuum isreleased, the freeze-dried blocks are removed, and are then split toremove the top and bottom surface layers, and to divide the remainder ofthe blocks into 3 mm-thick pads. The step of splitting the freeze-driedblocks into pads is carried out with a Fecken Kirfel K1 slitter.Finally, the pads are die-cut to the desired size and shape on adie-cutter, packaged, and sterilized with 18-29 KGy of cobalt 60gamma-irradiation. Surprisingly, this irradiation does not causesignificant denaturation of the collagen, which appears to be stabilizedby the presence of ORC. The resulting freeze-dried collagen ORC padshave a uniform, white, velvety appearance. The thickness of the pads is3.2±0.17 mm (N=8 batches).

Example 2

The procedure of Example 1 was repeated, but with addition of NAC andascorbate-2-triphosphate in amounts of 2.5 mM and 2.5 mM respectively tothe slurry.

Example 3

The procedure of Example 1 was repeated, but with addition of NAC andascorbate-2-triphosphate in amounts of 0.5 mM and 0.5 mM respectively tothe slurry.

Reference Examples 4-9

Reference sponges were prepared by the method of Example 1 with thefollowing actives in the slurry in the following amounts:

Reference Example 4: NAC

Reference Example 5: NAC 5 mM

Reference Example 6: NAC 9 mM

Reference Example 7: ascorbate-2-triphosphate 1 mM

Reference Example 8: ascorbate-2-triphosphate 5 mM

Reference Example 9: ascorbate-2-triphosphate 9 mM

Procedure 1; Measurement of Collagen Synthesis by Dermal Fibroblasts

A collagen synthesis assay with dermal fibroblasts was performed. Thisis standard assay which shows the amount of collagen synthesised byfibroblasts after stimulation with the active agents.

Briefly, the collagen synthesis assay involved plating 8.4×10⁴ humanfibroblasts (per well) into 24-well plates, and incubating them at 37°C., 5% CO₂, in 10% FBS-DMEM. Once cells were confluent (within 24 hoursof plating), the 10% FBS-DMEM was removed, cells washed 3× with SF-DMEM,before a SF-DMEM extract of the sponge samples of Examples 1-9 was addedto the cells. Cells were incubated for 72 hours after which time themedia were collected. A commercial immunoassay (Metra-CICP Kit, Quidel,San Diego, USA) was used that measured the level of C-terminalpropeptide of Type-1 Collagen (CICP) present in the cell culture media.The level of CICP in the media, which is released by the fibroblasts asa by-product of collagen synthesis, is proportional to the level ofcollagen synthesis and so its level was used to determine the level ofcollagen synthesis.

The results are shown graphically in FIG. 2. It can be seen that thecombination of ascorbate-2-triphosphate with NAC gives a synergisticimprovement in collagen synthesis. This is expected to result inimproved wound healing in vivo, in particular for chronic wounds.

Procedure 2: Measurement of Collagen Gel Contraction

The measurement of collagen gel contraction, performed in vitro, gives agood indication of the ability of actives to promote cellular response.The procedure was as follows:

1. Normal human dermal fibroblasts were maintained in 10% FBS/DMEM(Fetal bovine serum & Dulbecco's minimally modified medium), and grownin a humidified incubator containing 5% CO₂. Cells were split at 95%confluency and were used for experiments when approx. 90% confluent.2. Cells were harvested using 0.05% trypsin/EDTA (GIBCO BRL), countedusing a haemocytometer and centrifuged to obtain cell pellet.3. Cells were then resuspended at a cell density of 140,000 cells/ml—4times the final cell density in the collagen gel.4. The following cell/collagen mixture was prepared for each 24-wellplate:14 mls 10% FBS/DMEM7 mls cells at 140,000 cells/ml7 mls soluble collagen—rat tail type I collagen from CollaborativeBiomedical (supplied by Fred Baker Scientific: 356236)—finalconcentration 1 mg/ml—it is usually supplied at approx. 4 mg/ml thus 7mls was taken directly from the bought stock solution. This mixture wasthen distributed at 1 ml/well into each of a 24-well tissue cultureplate, and allowed to gel for 1 hr at 37° C.5. Once the gels had polymerized they were rimmed with a sterile pipettetip and an additional 0.5 ml of medium added carefully to each well.This additional medium contained the test reagent (at 3 times the finalrequired concentration to account for dilution).

The plates were photographed and then incubated at 37° C. in 5% CO₂ in ahumid environment. Photographs were taken at similar times each dayuntil the end of the experiment—usually day 14 or 15 after setup. At dayfour the measurements of each well for each test substance were averagedand were compared to each other and to the control. A faster the rate ofcontraction was indicated by a reduction in diameter of the gel.

Collagen gel contractions were prepared and incubated over four dayswith the following solutions. Positive control; 10% FBS DMEM—Negativecontrol; Serum Free DMEM. Test Solutions, 9 mM AZP, 9 mM NAC, 9 mMAZP/NAC combination and 4.5 mM NAG/AZP combination.

The results show that a faster rate of contraction is achieved with the4.5 mM AZP/NAC combination that with any other solution. This is afaster rate than the ascorbate-2-triphosphate or the NAC on their ownand is faster than the positive control. This indicates that this levelof active is optimal for the contraction of collagen and proliferationof the dermal fibroblasts.

All patent applications referred to herein are expressly incorporated intheir entirety.

The above examples have been described for the purpose of illustrationonly. Many other embodiments falling within the scope of theaccompanying claims will be apparent to the skilled reader.

The invention claimed is:
 1. A wound dressing, comprising: an activelayer comprising a wound-facing side, N-acetyl cysteine or a saltthereof, and a stabilized ascorbate, wherein a weight ratio of N-acetylcysteine or a salt thereof to the stabilized ascorbate is from about 1:4to about 4:1; a backing sheet extending over the active layer oppositeto the wound facing side; and a moisture-vapor transmitting adhesivelayer disposed between the backing sheet and the active layer; whereinthe stabilized ascorbate comprises a member selected from the groupconsisting of ascorbate 2-phosphate or polyphosphate compounds,trisodium L-ascorbyl-2-monophosphate, 2-phospho-L-ascorbic acidtrisodium salt, magnesium ascorbyl phosphate (MAP), L-ascorbic acidmono(dihydrogen phosphate) magnesium salt, magnesium L-ascorbicacid-2-phosphate, trisodium L-ascorbyl-2-polyphosphate, and acombination of any two or more thereof.
 2. The wound dressing of claim1, further comprising an absorbent layer positioned between the activelayer and the backing sheet.
 3. The wound dressing of claim 1, whereinthe active layer further comprises a carrier material.
 4. The wounddressing of claim 3, wherein the carrier material is bioabsorbable andadapted to provide sustainable release of N-acetyl cysteine or a saltthereof and the stabilized ascorbate.
 5. The wound dressing of claim 3,wherein the carrier material consists essentially of a freeze-driedsponge or a solvent-dried sponge.
 6. The wound dressing of claim 5,wherein the carrier material comprises oxidized cellulose, incombination with collagen or chitosan.
 7. The wound dressing of claim 1,further comprising a liquid-permeable top sheet disposed on the woundfacing side of the active layer.
 8. The wound dressing of claim 1,wherein the moisture-vapor transmitting adhesive layer is asubstantially continuous layer of pressure-sensitive polyurethaneadhesive coated on the backing sheet.
 9. The wound dressing of claim 1,wherein the backing sheet comprises substantially closed-cellpolyurethane foam.
 10. The wound dressing of claim 1, wherein thebacking sheet is larger than the active layer and extends beyond one ormore edges of the active layer to form a marginal region.
 11. The wounddressing of claim 1, which is sterile and packaged in amicroorganism-impermeable container.
 12. The wound dressing of claim 1,wherein the stabilized ascorbate comprises ascorbate-2-triphosphate. 13.The wound dressing of claim 3, wherein the active layer comprises fromabout 1 wt. % to about 10 wt. % of N-acetyl cysteine or salts thereof,and from about 1 wt. % to about 10 wt. % of the stabilized ascorbate.